Concrete, composed of cement, aggregate and water, is a well known building material having considerable compressive strength. There are multiplicities of application where low density concrete is a suitable, useful or desirable material since it has the advantage of light weight and favorable insulation properties.
In general there have been several methods to produce such low density concrete and lightweight aggregate. In one way, lightweight aggregate material, such as cinders available in ash heaps from coal-burning power plants, was used to produce such low density concrete products. However, a decade ago or more, when such cinders were no longer generally available, manufacturers substituted bloated slate, clays and shale, fly ash, pumice and the like which they produced in rotary kilns or sintering machines. While such kilned or sintered materials and methods using such heat-expanded materials are still currently in use, they are not very satisfactory or efficient as well as being increasingly very expensive due to material costs, fuel costs and labor costs. The expensive massive kilns or sintering equipment produce only relatively small amounts of product per working shift. Moreover, such heat-expanded aggregate making methods have not produced products with uniformly satisfactory properties. Besides requiring expensive and cumbersome machinery, heat-expansion processes create highly undesirable air pollution. Additionally, the specialized raw materials for producing such heat-expanded products are only available in certain limited geographic areas, often remote from the desired site for use.
Another manufactured lightweight aggregate is expanded slag. Hot dross is separated from the molten iron in steel production and is put in contact with water to cause bloating. Since the residue is a by-product, the aggregate is economical, but since it is dross it is neither uniform nor stable and therefore does not produce sufficiently uniform low density concrete.
Additionally, it has been suggested that low density concrete could be produced by making a "cellular concrete" by adding air-bubble containing foam to a concrete mix and trapping the air-bubbles therein. However, much of these bubbles are generally lost during the step in which the foamed composition is mixed with the concrete or during pouring of the concrete mix. The foamed compositions tend to break down or bubbles collapse and are lost during mechanical mixing of the compositions resulting in a large loss of air. Additionally, bubbles of the foamed composition tend to coalesce into each other and form relatively large and unstable air pockets, resulting in loss of cell integrity. Moreover, such cellular concretes have generally suffered from undesirable, unpredictable shrinkage and cracking during the curing or setting operation which tends to be erratic. All these factors tend to produce weakened cellular concrete. Also, such cellular concrete requires specialized on-the-job mixing apparatus, and the foam mix specifications must be tailorized for the necessary foam fluidity characteristics with increased water content needed to avoid undue loss of bubbles, rather than for the ultimate desired low-slump structural concrete mix specifications. Accordingly, such cellular concrete has found use primarily only in floor fills and roof deck applications, providing insulation and some modicum of fire protection, but due to the shrinkage and cracking or due to the need for specialized apparatus and the foam mix characteristics as described, conventional foamed concrete is generally unsuitable for use as a structural concrete.
One method of attempting to produce lightweight aggregate has been to provide a body of cured cellular concrete, breaking the body into fragments, coating the fragments with a thin layer of cement which is allowed to cure and incorporating the coated fragments in a cement matrix to form low density concrete. Such a method is disclosed for example in U.S. Pat. No. 4,351,670 issued Sep. 28, 1982 to Harold E. Grice. However, such products are not sufficiently stable and require a cumbersome process for preparation. In addition, such cellular concrete suffers from erratic curing or setting that results in setting-shrinkage or coalescing of cells and loss of cell integrity as discussed previously.
Moreover, the use of such cellular concrete to produce lightweight aggregate by heretofore employed methods has required the use of massive crushing equipment to transform the cellular concrete into suitable lightweight aggregate.
Another method of producing lightweight aggregate has been to add a colloidal solution or sol-gel of sodium bentonite, peptized calcium bentonite, attapulgite or a gelled silica, such as a sodium silicate CaCl.sub.2 sol-gel, to a cellular concrete mix. Such a method is disclosed in U.S. Pat. No. 4,900,359 issued Feb. 13, 1990, to Lawrence F. Gelbman. Setting-shrinkage in the resulting cellular concrete is substantially eliminated as is cell coalescing so that cell integrity of the cellular concrete is maintained. This particular cellular concrete is characterized by an increased strength to weight ratio and therefore is much more suitable for use in structural applications as well as for insulation purposes. Additionally, this cellular concrete absorbs substantially no water in the cells since the cells have not coalesced and are not interconnected. Moreover, such cellular concrete may be converted to substantially uniform and stable lightweight aggregate by heretofore known crushing methods. However, such cellular concrete is still quite expensive to produce in commercial quantities because cellular concretes of this type require substantial quantities of cement, generally the costliest ingredient in cellular concrete.
It is therefore an object of this invention to provide an economically manufactured lightweight aggregate that can be produced in commercial quantities to meet the increasing demand.